The present invention relates to a semiconductor laser, and in particular to a distributed-feedback semiconductor laser.
Semiconductor lasers have valence bands where electrons break away from atoms and create holes, and conduction bands where electrons move around freely. Valence bands form continuums of a low energy level, while conductors have a high energy level. When a positive valence band hole and a negative conductor electron re-bond, light is emitted, and a laser beam is generated. In other words, the movement of an electron from a high energy level to a low energy level causes light to be emitted in an active layer several microns thick in the junction area of the conductor and the valence band. Double heterojunctions have been developed, allowing different types of conductor to participate in double bonds so as to generate laser beams continuously at room temperature. Moreover, techniques have been developed whereby the cleavage planes of semiconductor laser devices are protected by a thin film, and long-life semiconductor lasers have been put to practical use.
The coherent nature of laser beams renders them eminently suited to use in fields as diverse as those of spectroscopy, instrumentation, optical communications, printing, optical discs and chemistry. In optical communications systems in particular, numerous improvements have been made in order to relay large amounts of data at high speeds over long distances. It is desirable that laser devices used as light-emitting elements in optical communications should transmit stably on a single wavelength. For instance, in the case of long-distance high-speed optical communications employing InGaAsP lasers with a wavelength of 1.55 micrometers, even relaxation oscillations require to be generated on a single wavelength because of the considerable material dispersion of the optical fibres.
Distributed-feedback lasers are an apparatus of securing single-wavelength oscillations. Within faces which are perpendicular to the optical axis of the laser beam, standing waves are created in perpendicular and parallel directions to the active layer. The perpendicular transverse mode comes about as a result of the light-entrapping effect of a double heterojunction, while the parallel transverse mode results from various stripe configurations.
Standing waves which are created between reflective surfaces are known as longitudinal mode. When the longitudinal mode changes, so does the oscillation wavelength, and it proves impossible to achieve single-wavelength oscillation. Distributed-feedback semiconductor laser devices, on the other hand, are provided with diffraction gratings which are characterised by cyclic changes in refractivity along the active layer. The presence of these diffraction gratings means that part of the optical waves which leak from the active layer are reflected cyclically. Generally speaking, diffraction gratings are fashioned in the form of irregularities at a prescribed pitch in the guide layer or clad layer which adjoins the active layer above or below. Thus, optical waves proceeding parallel with the diffraction gratings are partially reflected at a prescribed angle perpendicular to the direction of reflection. This is how single-mode oscillation has been achieved.
One example of this type of semiconductor laser is the .lambda./4 phase shift distributed-feedback semiconductor laser (hereinafter referred to as `.lambda./4 phase shift laser`). This .lambda./4 phase shift laser achieves single-mode (single-wavelength) oscillation by virtue of Bragg wavelengths induced by diffraction gratings. In .lambda./4 phase shift lasers of this sort, there is a tendency for the electric field to converge on the phase-shifting diffraction grating if the normalized coupling coefficient (k) is great. This is inconvenient because it leads to instability of single-mode oscillation at times of high output.
Techniques for flattening the distribution of electric field intensity in the axial direction of the resonator in this phase shift laser so as to ensure stability of oscillation in single mode have been suggested, for example, in Japanese Patent Publication No. A-4-100287, or by Okai et al. in IEEE J. Quantum Electronics Vol. 27, pp. 1767-72 (1991).
The proposed semiconductor laser is of the configuration illustrated in FIG. 10, where two types of diffraction gratings with differing pitchs are formed within the resonator (laser element) as shown in FIG. 11. This type of distributed-feedback semiconductor laser will be referred to hereinafter as a cyclic modulation laser.
In FIG. 10, a diffraction grating 12 is formed on a semiconductor substrate 11 for the purpose of selecting the oscillation wavelength. On the semiconductor substrate 11 on which is formed the diffraction grating 12 are also formed by MOPVE (metal-organic vapour phase epitaxy) growth an optical guide layer 13, an MQW (multi-quantum well) active layer 14 with gain, and a clad layer 15 in that order. Sandwiching this MQW active layer 14 between the n-type semiconductor substrate 11, which has a large band gap, and the p-type semiconductor clad layer 15 allows the formation of a double heterostructure (DH).
Referring to FIG. 11, the diffraction grating is such that the areas at both ends of the laser element are formed with equal and uniform pitch or cycle (uniform pitch areas 18), while the central area has a shorter pitch or cycle (phase adjustment area 19).
On the surface of the semiconductor substrate 11 and the clad layer 15 are each located electrodes 17, the purpose of which is to inject electrons from the semiconductor substrate 11 into the MQW active layer 14, while to both ends of the laser element is applied an AR coating (non-reflective coating) 16.
Here, the sum-total of the amount of phase change induced by the diffraction grating of the phase adjustment area 19 in relation to the phase of the diffraction grating of the uniform pitch areas 18 is defined as the amount of cumulative phase change.
In the cyclic modulation laser illustrated in FIG. 10, the diffraction grating of the phase adjustment area 19 is constructed in such a manner that the amount of cumulative phase change is one half of the pitch of the diffraction grating of the uniform pitch areas 18. That is to say, in the example illustrated in FIG. 10, the diffraction grating of the phase adjustment area 19 is constructed so as to be one quarter of the wavelength (.lambda./4) in relation to the standing wave of the laser beam within the resonator.
Conventional distributed-feedback semiconductor lasers, having had within the laser element diffraction gratings with two differing types of pitch, have been problematic in that it has been impossible to obtain a sufficient difference (margin) in oscillation threshold gain between the principal and subordinate modes, especially where the injected current is small as in the vicinity of the threshold value.
They also present a problem in that since, as has been explained, the diffraction grating of the phase adjustment area is constructed in such a manner that the amount of cumulative phase change is one half of a pitch of the diffraction grating of the uniform pitch areas, there is a limit to the degree of flattening of distribution of electric field intensity in the axial direction of the resonator (the horizontal direction in FIG. 10) which is feasible, so that it is not necessarily possible to achieve stable single-mode oscillation.